This invention relates generally to convenience food service techniques in which a meal is first cooked, then refrigerated and stored, and subsequently reheated; and more particularly to a system for use on aircraft for reheating pre-cooked meals without degrading the basic texture, flavor and nutritional qualities of the meal.
In long distance commercial airline flights, it is customary to provide passengers with a hot meal. To this end, the present practice is to cook the meals in a commissary, the meals being placed in sealed trays and then refrigerated. These trays are thereafter transported to airports where they are loaded before flight into partitioned lockers on planes. Electrical heating elements are incorporated in the partitions so that the lockers serve as ovens for reheating the refrigerated meals to a temperature suitable for serving.
Though airlines generally employed experienced and skilled chefs to supervise the en masse preparation of meals in commissaries and care is exercised in choosing basic ingredients of good quality, it is nevertheless a fact that most meals served by airlines are at best of mediocre quality. Thus the typical meat dish leaves much to be desired in terms of appearance, taste and flavor. Certainly a steak served on a plane is a far cry from a steak served in a first-class restaurant, even though in both cases, use is made of meat of prime quality. This is also true of vegetables, which when originally cooked are of good, acceptable quality, yet when served in an airline tray are often overcooked and unappetizing.
In reheating a pre-cooked meal, it is difficult, when going from the refrigerated state to an adequately heated service condition, to avoid a situation in which the core of the product is still cold even though the outer layer is quite hot. And when one seeks to ensure that the body of the food is hot throughout, there is a tendency to overheat the meal and thereby re-cook it with a resultant loss of nutritional value and flavor.
An electrically-operated food locker in an aircraft functions essentially as a conventional reheating oven and suffers from serious heat transfer and control problems. In such ovens, one necessarily goes through a heat-up phase during which the temperature of the pre-cooked meal must be raised from its initial cold state to a service temperature level, at which point the food-loaded trays must be maintained at a service temperature level until such time as the trays are removed from the locker and brought by attendants to the seats of passengers.
During the heat-up phase, the rate of heat transfer from the heated air in the locker to the relatively cold food-loaded trays depends on the temperature differential; the greater the difference between the air temperature and the food temperature, the more rapid the rate of heat transfer. Since the hot air temperature throughout the locker is at a fairly uniform level, the transfer rate at the outset of heating in the heat-up phase is very rapid; but as the difference in temperature between the hot air and the food thereafter diminishes, the rate of transfer becomes increasingly slow and quite sluggish as the service temperature is approached.
Assuming that the food in the trays is initially at a temperature of about 10.degree. F. and that it is necessary to raise the food temperature to a service level of about 150.degree. F., and further assuming a hot air temperature of about 165.degree. F. in the locker to avoid re-cooking, then at the outset of the heat-up phase, there will be a sharp differential giving rise to very rapid heating. But as this temperature differential diminishes in the course of the heat-up phase, the rate of heat transfer slows down. When, for example, the food temperature reaches 130.degree. F., then the temperature differential relative to the heated air is only 35.degree. F., and it takes a relatively long time before the food temperature can be raised to the service temperature of 150.degree. F., at which point the heat-up phase is conducted and a service phase is initiated.
Thus if one plots a curve of food temperature (10.degree. F. to 150.degree. F.) vs. time in the heat-up phase, the resultant curve for a hot-air temperature of 165.degree. F. will exhibit a sharp rise from 10.degree. F. to about 100.degree. F. within a fairly short time interval, the curve thereafter leveling off as the temperature goes more gradually from 100.degree. F. to 150.degree. F. As a result, the duration of the heat-up phase is unduly prolonged, which in some situations may be a practical disadvantage. If, for instance, the aircraft lockers are loaded with cold food trays which must be made available for service to passengers in about one half-hour after loading, this time may be inadequate to bring the meals to the proper service level.
On the other hand, while on a given flight the waiting period before meals are to be served may be adequate for the heat-up phase, because of unpredictable flight delays it may be several hours before the meals are actually served to passengers. Thus the phase during which food must be maintained in the lockers at a service temperature, instead of being, say, a half-hour period, may run for three or four hours or even longer.
As a practical matter, with electrically-operated ovens, it is difficult to maintain food at a substantially constant service temperature level for a prolonged period. Consequently, should there by an unexpected flight delay, the meals in the lockers may be recooked, dried out, and rendered unpalatable.
In my above-identified copending application of which the present application is a continuation-in-part, there is provided a system adapted to rapidly raise the temperature of a pre-cooked meal from a cold or frozen state in a manner bringing the internal temperature of the entire body of the meal to substantially the same predetermined elevated service temperature level, the meal thereafter being maintained indefinitely at the service temperature level without recooking.
In the system disclosed in my copending application, during the heat-up phase the cold pre-cooked meals are raised in temperature by periodic pulses of hot air which flow past the trays at high velocity, the temperature of the pulses being well above the predetermined elevated service temperature level, whereby the temperature differential between the heated air and the food is high even when the food approaches the service temperature level, thereby effecting a high rate of heat transfer and causing the food to reach the service temperature level quickly without, however, excessive heating thereof.
To this end, there is provided a thermally-insulated chamber having a receiving station for the trays flanked by input and output plenums. A main flow loop is provided in which the chamber is connected in a continuous flow path in series with heater station and an air pump or blower in an arrangement in which air drawn via an output line from the output plenum and creating a negative pressure therein is conducted through the heater station and then forced in the heated state through an input line leading into the input plenum to create a positive pressure therein. The resultant pressure differential between the plenums causes heated air to flow at high velocity through the section to heat the meals contained therein.
A by-pass extending between the input to the heater station and the junction of the chamber and the pump in the pump flow line defines a feedback flow loop which excludes the chamber. A damper mechanism at this junction is cyclically driven to periodically block the flow of heated air through the main loop into the chamber and to divert the flow into the feedback loop for recirculation therein.
As a consequence, main loop flow through the chamber assumes the form of a pulsatory wave whose fluidic pulses have a peak temperature whose level is well above the pre-determined temperature level and whose relaxation periods are at a temperature below this predetermined level, thereby promoting rapid heat transfer in the body of the food without, however, raising the surface temperature thereof above this level. This action is continued until the entire body of the product is at the desired service temperature level, at which point the system is operated in a service phase to maintain this level indefinitely without overheating the food product.
The pulsating hot-air system disclosed in my copending patent application will function in any environment including an airplane. However, the typical plane has very limited space available for accommodating food storage and heating facilities. It is necessary, therefore, to design a pulsating hot-air system which will operate effectively within these constraints.